I guess I wasn't sufficiently clear in the post, but the part I think is interesting is not that tmpfs and SSD bench at the same speed. I am aware of in-memory

I guess I wasn't sufficiently clear in the post, but the part I think is interesting is not that tmpfs and SSD bench at the same speed. I am aware of in-memory filesystem caches, and explicitly mention them twice in the last few paragraphs. The interesting part, to me, was that using the vfs crate or the rsfs crate didn't produce any differences from using tmpfs or an SSD. In theory, those crates completely cut out the actual filesystem and the OS entirely. Somehow, avoiding all those syscalls didn't make it any faster? Not what I expected. Anyway, if you have examples of in-process filesystem mocks that run faster than the in-memory filesystem cache, I'd love to hear about them.